Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share [Abstract]
Net income	$ 21,375	$ 24,169	$ 89,969	$ 82,108
Weighted average shares outstanding: [Abstract]
Basic (in shares)	46,574	47,987	47,076	48,357
Effect of dilutive securities: [Abstract]
Stock options and employee stock purchase plan (in shares)	1,213	785	1,129	800
Diluted (in shares)	47,787	48,772	48,205	49,157
Basic (in dollars per share)	$ 0.46	$ 0.50	$ 1.91	$ 1.70
Diluted (in dollars per share)	$ 0.45	$ 0.50	$ 1.87	$ 1.67
Anti-dilutive stock options excluded from diluted earnings per share computations (in shares)	4	1,418	964	1,418